There have been proposed focus detection apparatuses for a biological microscope such as a technique that keeps focusing by detecting a glass interface in the vicinity of a sample using reflection of infrared light (for example, see Japanese Patent Application Laid-Open No. 2004-070276) is extremely effective upon performing an observation together with a dosage or a long time observation, or a continuous observation of a plurality of positions of a sample.
However, in a recent biological microscope, wavelength of a fluorescent dye used upon fluorescence observation has been getting longer, and such an observation using optical tweezers or two-photon excitation has been adopted. Accordingly, in a biological microscope, the wavelength of observation light is not limited to visible light, so that light having a wavelength range of near-infrared cannot be exclusively used for focus detection and keeping thereof.